net/libproxy: unbreak build with Clang 6 (C++14 by default)

libproxy/test/get-pac-test.cpp:48:10: error: assigning to 'int' from incompatible type '__bind<int &, sockaddr *, unsigned long>'

ret = bind(m_sock, (sockaddr*)&addr, sizeof (struct sockaddr_in));
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported by: antoine (via bug 224669)